I want to type an array where the first element is always a number, and the rest of the elements are always strings, and the length as to be at least 1.
Here is my best effort:
type MyArray =
| [number]
| [number, string]
| [number, string, string]
| [number, string, string, string];
How do I make that go on forever?
Before TypeScript 3.0 there really wasn't anything better than a union like you have. But then a bunch of related new features were introduced to better support tuple types, and specifically their relationship to lists of function parameters. And since functions support a final rest parameter representing the type of an indefinite number of parameters as an array, it made sense to introduce a final rest element in a tuple representing the type of an indefinite number of tuple elements as an array.
Hey, in fact, your use case is explicitly mentioned as an example in the documentation:
For example,
[number, ...string[]]means tuples with anumberelement followed by any number ofstringelements.
So let's try that:
type MyArray = [number, ...string[]];
const okay0: MyArray = [0]; // okay
const okay1: MyArray = [1, "a"]; // okay
const okay2: MyArray = [2, "a", "b"]; // okay
const okay3: MyArray = [3, "a", "b", "c", "d", "e", "f", "g", "h", "i", "j"]; // okay
const bad0: MyArray = [0, "a", false]; // error!
// ~~~~ <-- boolean is not assignable to string
const bad1: MyArray = ["x", "y"]; // error!
// ~~~ <-- string is not assignable to number
const bad2: MyArray = []; // error!
// ~~~~ <--- property '0' is missing (i.e., bad2[0] is missing)
Looks good to me. Hope that helps; good luck!
(number | string)[]and helper functions to extract thenumberfrom the first item andstrings from the rest, or what I would do is use an object type that has{ foo: number, bar: string[] }[number, ...string[]]will work in recent versions of TypeScript.